The incommensurate modulated structure of Bi1.91Sr2CoO5.93 (a = 5.4614(6) Å, b = 5.4471(5) Å, c = 23.415(2) Å) with * = 0.240(2)*+* has been refined in the superspace group F2mm(01)00s using the four-dimensional formalism for modulated structures and single-crystal x-ray diffraction data. Two different regions with different sites for Bi and O atoms are evident along the modulation direction (called M and I regions). The oxygen disorder observed in the M region leads to different possible configurations for [BiO] layers. In particular, a double-chain configuration comparable to the one observed in related compounds is outlined. Therefore, a comparison with related compounds (BiCu or BiFe) is performed.

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Two single crystals of the Pb-doped high-T c superconductor Bi 2Ϫx Pb x Sr 2 CaCu 2 O 8ϩ␦ phase have been isolated with xϭ0.4 and 0.2. In the first case, the structure is monoincommensurately modulated with the so-called Pb-type modulation (q II ϭ0.11b à ). In the second case, the structure is characterized by a double modulation (q I ϭ0.2b à ϩc à ; q II ϭ0.15b à ). The two modulated structures have been refined from singlecrystal x-ray diffraction data using the four-and five-dimensional superspace groups' formalism, respectively. Considering these substituted phases and the well-known lead-free phase, the same symmetry is observed for their average structures, while their structural modulations exhibit significant discrepancies. Different disordered regions are characterized in relation to the modulation period. The results are confirmed by highresolution electron-microscopy observations. The relative structures are compared with each other and the structure of the lead-free phase, particularly about the stacking scheme of the structural layers.FIG. 1. Schematic representation of diffraction pattern corresponding to ͑a͒ modulation I ͑dark circles; first-and second-order satellite reflections͒, ͑b͒ double modulation ͑stars represent intermodulation satellites͒, and ͑c͒ modulation II ͑open diamonds; firstand second-order satellite reflections͒.

Structures refined for two members of the group of Bi2-xPbxSr2CoO6-type compounds are proposed. For x = 0.16, the structure is incommensurately modulated (a = 5.453(2) Å, b = 5.426(2) Å, c = 23.43(1) Å, q⃗⋆ = 0.245(2)b⃗⋆ + c⃗⋆, F2{mm}(0β1)00{s}). For x = 0.8, the modulation becomes very weak and has been neglected in the structural refinement (a = 5.3117(7) Å, b = 5.4232(6) Å, c = 23.590(2) Å, Pnan). A new interpretation of the undoped modulated Bi2Sr2CoO6 phase is given. A new type of configuration for the [BiO] layers is confirmed and original modulation features are elucidated in comparison with the related superconducting cuprate phases.